1. Field of the Invention (Technical Field)
The present invention relates to a dielectric substrate useful in the manufacture of printed wiring boards wherein the dielectric substrate comprises at least one organic polymer having a Tg greater than 140xc2x0 C. and at least one filler material. The dielectric substrate of this invention has a dielectric constant that varies less than 15% over a temperature range of from xe2x88x9255 to 125xc2x0 C. This invention also includes multilayer printed circuit boards including dielectric substrates of this invention and especially printed circuit boards wherein the dielectric substrate of this invention is used to form internal distributed capacitors in the printed circuit board.
2. Background Art
Multilayer printed wiring boards (PWB""s) are widely used in electronic devices such as computers, telephones, appliances, automobiles and the like. Multilayer printed wiring boards typically include a board having a plurality of insulated conductive trace layers separated by dielectric substrate layers. Due to the demand for smaller and smaller electronic devices, efforts have been made to incorporate circuit devices directly into printed wiring boards. For example, U.S. Pat. No. 5,010,641 discloses a multilayer printed wiring board including internal distributed capacitors. Similarly, U.S. Pat. Nos. 5,155,655 and 5,161,060 disclose capacitor laminates useful in the manufacture of capacitive printed wiring boards.
The capacitors built into printed wiring boards typically include a metal ground layer and a charged metal layer divided by a dielectric substrate layer. The dielectric layers used in printed wiring board capacitors are generally polymeric sheets that may be reinforced with materials such as glass, ceramics and so forth. More recently, there has been a trend towards reducing the profile of circuits and dielectric layers in printed wiring boards in order to increase printed wiring board circuit density. This trend has resulted in the reduction of the thickness of dielectric layers associated with wiring board capacitors. However, a problem with reducing the thickness of capacitor dielectric layers is that the dielectric constant of the dielectric layer varies too widely with varying temperatures causing undesirable variable capacitance. Therefore, there remains the need for dielectric substrate layers that are useful in forming printed wiring board capacitors that are thin and have dielectric constants that vary little over a wide temperature range.
A primary object of the present invention is to provide a dielectric substrate that has a dielectric constant that varies little over a wide temperature range.
Another primary object of the present invention is to provide a dielectric layer that includes a non-sintered particular material.
Another primary object of the present invention is to provide a capacitor laminate including a dielectric sheet having an essentially unchanging dielectric constant which is located between conductive foil layers.
A primary advantage of the present invention is a composite comprising at least one organic polymer having a Tg greater than about 140xc2x0 C., and at least one ferroelectric particle filler. The organic polymer and ferroelectric particles are chosen to produce a composite having a dielectric constant that varies less than 15% when the composite is subjected to temperatures ranging from xe2x88x9255 to 125xc2x0 C.
Another primary advantage of the present invention is a capacitor laminate comprising a composite of this invention formed into a sheet and having a top surface and a bottom surface. The capacitor laminate includes a first conductive layer associated with the composite top surface, and a second conductive layer associated with the composite bottom surface.
Another primary advantage of the present invention includes multi-layer printed wiring boards wherein at least one layer comprises a capacitor laminate of this invention.